COS 198-1 - Overabundant deer and garlic mustard invasion destabilize niche differences and threaten coexistence of native understory plants

Background/Question/Methods

When the local context for species in a community is altered by dramatic or abrupt changes in interacting species abundances, the fitness and abundance of species within a community are predicted to change radically. Instances of such disruption include pollinator declines, overabundant herbivores, disturbance, more extreme abiotic factors, and the arrival of exotic invaders. Here, we aimed to disentangle the causal mechanisms and processes that underlie the demographic responses of native species experiencing shifts in biotic interactions to project how community composition may change through time.

We fused aspects of integrated population models with both Bayesian integral projection models and community-combined integral projection models to develop a synthetic community model that incorporates biotic effects on vital rates, i.e., growth, survival, and reproduction. With data spanning almost two decades, we parameterized vital rate functions for five long-lived forest understory plants whose co-occurring populations in Pennsylvania have experienced the impacts of overabundant deer and a highly invasive non-native plant, garlic mustard, that disrupts root-fungal symbionts. We measured the effect of deer and invasion via the experimental exclusion/removal of deer/garlic mustard in the field, and we also included negative density dependence among focal plants to measure competitive interaction strengths.

Results/Conclusions

Our results show negative effects of overabundant deer on most native plant vital rates, either by direct consumption or indirect environmental modification. The invasive competitor had less of an effect on plant vital rates. However, it did alter the strength of competition among native plants, which is in line with previous work that suggests garlic mustard disrupts root-fungal symbionts and reduces plant nutrient uptake. We found that native plants competed more strongly with conspecific neighbors than heterospecific neighbors, which indicates niche partitioning, but this pattern was not evident for the invader.

Together, our results support previous findings that overabundant deer in the Northeastern deciduous forest facilitate garlic mustard invasion. Our preliminary results suggest these co-occurring human-caused stressors will affect native plant coexistence and community composition. To further test this expectation, we are now working to project our community dynamics model to explore coexistence in four plausible future landscapes (+/- overabundant deer, +/- invasive competitor).

Our work demonstrates how existing approaches can be fused into one mechanistic process-based model that links individual responses to community dynamics. Explicitly linking population to community dynamics is increasingly important for understanding and predicting how on-going environmental change will influence community assemblage processes and ecosystem function.